Flexible open-celled polymeric foams are widely used for energy absorption or insulation (thermal, acoustic, mechanical), filtration, absorption of fluids, and the like. In most cases, the foams desired for these purposes have relatively homogeneous structures comprising cells within a given size range joined by open "windows" or "holes" to adjacent cells. Among the various measures of such foams that are important for each application are cell size/hole size and distribution, anisotropy, proportion of cell struts to windows, and porosity. Anisotropy can arise from the relative deviation between the shape of all the cells in the foam and some geometric ideal. In general, work has been devoted to making foams as homogeneous and isotropic as possible with respect to cell size and shape and density.
Polyurethane foams having a range of densities are known materials. For example, the "integral skin" flexible polyurethane foams have high density skin layers that transition gradually over 1-3 cm into a lower density core region. See for example Ashida, K.; Iwasaki, K. In Handbook of Plastic Foams", Landrock, A. H., ED.; Noyes, 1995; Chapter 2, pp 56, 64-67, incorporated herein by reference. The overall densities of such foams are typically between about 200 and 1,100 kg/m.sup.3. These foams do not exhibit distinct regions having different compositional or microstructural properties within a single piece. Typically, these foams also have higher densities, larger cell sizes and/or larger hole sizes than foams which may be preferred for certain applications.
Foams may be made from polymer networks which have been entangled to form an interpenetrating network (IPN). IPNs may exhibit some of the properties of both polymer types. See for example Odian, G. G. "Principles of Polymerization", 3rd edition, Wiley-lnterscience: New York, 1991, New York, pp 149-150. IPNs do not inherently relate to control over any features at a supramolecular scale (e.g., density or cell size).
Laminates or sandwiches of two or more layers of foams having differing properties are also well known. See for example, Gibson, L. J.; Ashby, M. F. "Cellular Solids" Pergamon Press: Oxford, 1988, Chapter 9. Formation of such composites requires an additional step and may require use of adhesive which may interfere with the functioning or weight of the foam composite and serves as a potential point of failure.
The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and sanitary napkins, is the subject of substantial commercial interest. The ability of such products to acquire, distribute, and store fluids such as are found in body exudates (e.g., urine, sweat, feces, and menses) is obviously critical to their function. Historically, this has been primarily achieved by using a combination of cellulosic fibers and interspersed superabsorbent particles (generally lightly crosslinked partially neutralized polyacrylic acid that forms a gel when exposed to free water). This approach has, however, encountered a number of difficulties in achieving efficient removal of fluid from the body of the wearer and storage away from the wearer, in part due to the difficulty in controlling and maintaining the appropriate blend of particulate and fiber to provide the desired degree of capillary fluid transport and core integrity and flexibility.
Other absorbent materials capable of providing capillary fluid transport include certain types of polymeric foams in absorbent articles for the purpose of imbibing, wicking and/or retaining aqueous body fluids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,520 (Garvey et al), issued Apr. 26, 1988 (absorbent composite structure such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams). These foams can provide core integrity and flexibility but not the desired degree of capillary fluid transport.
The use of appropriate absorbent foams in absorbent articles such as diapers and catamenial pads can provide features of capillary fluid acquisition, transport and storage required for use in high performance absorbent cores. Absorbent articles containing such foams can possess desirable wet integrity, can provide suitable fit throughout the entire period the article is worn, and can minimize changes in shape during use (e.g., swelling, bunching).
Particularly suitable absorbent low density open-celled foams have been made from High Internal Phase Emulsions (hereafter referred to as "HIPEs"). See, for example, U.S. Pat. No. 5,260,345 (DesMarais et al), issued Nov. 9, 1993; and U.S. Pat. No. 5,268,224 (DesMarais et al), issued Dec. 7, 1993. Other suitable absorbent foams are described in co-pending applications U.S. Ser. No. 08/370,922 (DesMarais et al, filed Jan. 10, 1995); U.S. Ser. No. 08/370,695 (Stone et al., filed Jan. 10, 1995); U.S. Ser. No. 08/370,697 (Dyer, filed Jan. 10, 1995); and U.S. Ser. No. 08/520,793 (DesMarais, filed Aug. 30, 1995), all incorporated by reference herein. The HIPE process provides facile control over the cell and hole size and distribution, proportion of cell struts to windows, and porosity in these foams. When suitably treated to render the surface of these foams hydrophilic, these HIPE foams provide desirable fluid handling properties, including: (a) relatively good wicking and fluid distribution characteristics to transport the imbibed urine or other body fluid away from the initial impingement zone and into the unused balance of the foam structure to allow for subsequent gushes of fluid to be accommodated; and (b) a relatively high storage capacity with a relatively high fluid capacity under load, i.e., under compressive forces. These HIPE absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article, and can be made relatively thin until subsequently wetted by the absorbed body fluid.
An important issue associated with the fluid handling properties of an absorbent foam is capillary structure. Foams having larger cell sizes and hole sizes tend to acquire fluid quickly but do not distribute fluid sufficiently against the force of gravity, nor do they store fluid effectively. Conversely, foams having smaller cell sizes and hole sizes are able to wick fluid against the force of gravity and store the fluid tightly to keep it away from the skin of the wearer, but are typically slower to acquire fluid. As indicated, heretofore, these opposing functions in an absorbent article have been achieved primarily by layering, in the z-direction, different types of distinct absorbent foams which provide different functions. This adds complexity and cost to the process and can limit the product designs desired to those which allow for combination of individual pieces. Also, these designs are limited in being able to move fluids by differential capillary pressure only between the layers in the "z-dimension" of the articles, i.e., not in the "x-y dimension" or within the plane of a given layer.
Another important issue is the strength or resistance to compression deflection of the foam. Foams having comparatively higher densities, higher Tgs (defined hereinafter), and/or higher crosslinker levels generally exhibit greater resistance to deforming under pressure. This is achieved at the expense of using more polymer per unit volume, having a foam which deforms too slowly for practical use, or a foam which is too brittle or inflexible.
Accordingly, it would be desirable to be able to make an open-celled polymeric foam material that combines the various properties listed above as distinct regions within one material that: (1) expresses a combination of cell sizes and/or hole sizes in different distinct regions of a single piece of foam, and/or (2) expresses a combination of strengths and flexibilities in a single piece of foam, and/or (3) expresses a combination of energy absorbent properties in a single piece of foam. This can circumvent, at least partially, the need to use separate pieces of foams to serve different purposes, thereby enhancing the efficiency and simplicity of articles made with the objects of the current invention.